i would go with Satele Shan. She would have more experiance then Thanaton (as she started fighting Sith at the begining of the war... litteraly) She also has a messure of Revan's power in the Force as she is his decendent. She seems to wield the Force in a more personal and combative role, such as boosting her phisical powers rather than using it as it's raw form.
Thanaton was apprenticed to a Dark Council member befoe being named to his chair after defeating the Emperor's former apprentice. As i understand it Thanaton was only defeated by Nox because Nox had four Force spirits at his disposal.
So i the end it's a very close fight. the question would be what arena and who had the advantage on the battle field. But in the end i would say Satele. She was named Grand Master for a reason. :P